1. Field of the Invention
The present invention relates to a layered chip package that includes a plurality of chips stacked and a heat sink.
2. Description of the Related Art
In recent years, a reduction in weight and an improvement in performance have been demanded of mobile devices typified by cellular phones and notebook personal computers. Accordingly, there has been a demand for higher integration of electronic components for use in mobile devices. Higher integration of electronic components has been demanded also for achieving an increase in capacity of semiconductor memory.
As an example of highly integrated electronic components, a system-in-package (hereinafter referred to as SiP), especially an SiP utilizing a three-dimensional packaging technology for stacking a plurality of chips, has attracting attention in recent years. In the present application, a package including a plurality of chips stacked is called a layered chip package. Since the layered chip package allows a reduction in wiring length, it provides the advantage of allowing a higher operation speed for a circuit and a reduction in stray capacitance of wiring, as well as the advantage of allowing higher integration.
Major examples of three-dimensional packaging technology for fabricating a layered chip package include a wire bonding method and a through electrode method. According to the wire bonding method, a plurality of chips are stacked on a substrate and a plurality of electrodes formed on each chip are connected, by wire bonding, to external connecting terminals formed on the substrate. According to the through electrode method, a plurality of through electrodes are formed in each of chips to be stacked and inter-chip wiring is performed through the use of the through electrodes.
The wire bonding method has a problem that it is difficult to reduce the distance between the electrodes so as to avoid contact between wires, and a problem that high resistances of the wires hamper a high-speed operation of a circuit.
The through electrode method is free from the above-mentioned problems of the wire bonding method. Unfortunately, however, the through electrode method requires a large number of steps for forming the through electrodes in chips, and consequently increases the cost for the layered chip package. According to the through electrode method, forming the through electrodes in chips requires a series of steps as follows: forming a plurality of holes for the plurality of through electrodes in a wafer that will be cut later to become a plurality of chips; forming an insulating layer and a seed layer in the plurality of holes and on the top surface of the wafer; forming a plurality of through electrodes by filling the plurality of holes with metal such as Cu by plating; and removing unwanted portions of the seed layer.
According to the through electrode method, the through electrodes are formed by filling metal into holes having relatively high aspect ratios. Consequently, voids or keyholes are prone to occur in the through electrodes due to poor filling of the holes with metal, so that the reliability of wiring using the through electrodes tends to be reduced.
According to the through electrode method, an upper chip and a lower chip are physically joined to each other by connecting the through electrodes of the upper and lower chips by means of, for example, soldering. The through electrode method therefore requires that the upper and lower chips be accurately aligned and then joined to each other at high temperatures. When the upper and lower chips are joined to each other at high temperatures, however, misalignment between the upper and lower chips can occur due to expansion and contraction of the chips, which often results in electrical connection failure between the upper and lower chips.
U.S. Pat. No. 5,953,588 discloses a method of manufacturing a layered chip package as described below. In this method, a plurality of chips cut out from a processed wafer are embedded into an embedding resin and then a plurality of leads are formed to be connected to each chip, whereby a structure called a neo-wafer is fabricated. Next, the neo-wafer is diced to form a plurality of structures each called a neo-chip. Each neo-chip includes: one or more chips; resin surrounding the chip(s); and a plurality of leads. The plurality of leads connected to each chip each have an end face exposed at a side surface of the neo-chip. Next, a plurality of kinds of neo-chips are laminated into a stack. In the stack, the respective end faces of the plurality of leads connected to the chips of each layer are exposed at the same side surface of the stack.
Keith D. Gann, “Neo-Stacking Technology”, HDI Magazine, December 1999, discloses fabricating a stack by the same method as U.S. Pat. No. 5,953,588, and forming wiring on two side surfaces of the stack.
The manufacturing method disclosed in U.S. Pat. No. 5,953,588 involves a number of process steps and this raises the cost for the layered chip package. According to this method, after the plurality of chips cut out from the processed wafer are embedded into the embedding resin, the plurality of leads are formed to be connected to each chip to thereby fabricate the neo-wafer, as described above. Accurate alignment of the plurality of chips is therefore required when fabricating the neo-wafer. This is also a factor that raises the cost for the layered chip package.
U.S. Pat. No. 7,127,807 B2 discloses a multilayer module formed by stacking a plurality of active layers each including a flexible polymer substrate with at least one electronic element and a plurality of electrically-conductive traces formed within the substrate. According to this multilayer module, however, it is impossible to increase the proportion of the area occupied by the electronic element in each active layer, and consequently it is difficult to achieve higher integration.
If a layered chip package includes a chip that generates a large amount of heat during operation (hereinafter referred to as a heat generating chip), such as a microprocessor, the following problems arise. First, the heat generated by the heat generating chip can adversely affect the operation not only of the heat generating chip but also of other chips in the layered chip package. Second, if vertically adjacent chips in the layered chip package are bonded to each other with an adhesive, the heat generated by the heat generating chip can melt the adhesive, causing failures of bonding and electrical connection between the chips to thereby deteriorate reliability. A layered chip package including a heat generating chip therefore requires that the heat generated by the heat generating chip be dissipated.
U.S. Pat. No. 5,883,426 discloses a stack module that includes a plurality of substrates stacked, each substrate having a semiconductor chip mounted thereon, with a wave-shaped heat-dissipating member or heat pipe provided between the semiconductor chip and the substrate adjacent thereto. Published Unexamined Japanese Patent Application No. 2000-252419 discloses a three-dimensional module that includes a plurality of devices (semiconductor packages) stacked on a substrate, with heat sinks provided between the substrate and a device and between every vertically adjacent two devices, respectively.
A layered chip package including a heat generating chip may also be provided with heat sinks between every vertically adjacent chips, like the modules disclosed in U.S. Pat. No. 5,883,426 and Published Unexamined Japanese Patent Application No. 2000-252419. In this case, however, it is necessary that the heat sinks be extended horizontally in order to obtain a sufficient heat dissipation effect. This gives rise to the problem that the layered chip package including the heat sinks increases in size. Besides, in a layered chip package wherein inter-chip wiring is established by the through electrode method, it is impossible to provide heat sinks between every vertically adjacent chips.